The present invention relates to a generator, to a magnetic flux conducting unit for a generator, and to a power generation machine comprising such a generator. In particular, but not exclusively the present invention relates to a direct drive generator and to a magnetic flux conducting unit for a direct drive generator.
In the field of electricity generation, it is well known to provide a generator coupled to a fluid driven turbine such as those found in oil, gas, coal and nuclear power stations. Conventional generators comprise a rotor having an iron core with a number of current-carrying coils wound on the core, and an iron-cored stator carrying a winding. A magnetic field is generated by passing a current along the rotor coils such that, on rotation of the rotor, a current is induced in the coils of the stator winding. The rotors of generators found in power stations are coupled to the turbine by a drive shaft which rotates at a high rotational velocity, of the order of several thousand rpm, and with a relatively low drive torque. Conventional power generators, manufactured with this in mind, have therefore been designed for high velocity, low torque operation.
In recent years, significant research has been conducted worldwide into sustainable electricity generation methods, including wind, wave and tidal power generation.
Existing wind machines comprise a prime mover in the form of a large diameter rotor. The rotor has a number of rotor blades, mounted on a rotor shaft, which is coupled to a power generator. The turbine rotor typically rotates at relatively low rotational speeds and at a high output torque, for example, 20 rpm for a 2MW machine, with an output torque of around 955 kNm. It will therefore be understood that turbines of this type operate at relatively low rotational speeds, but with a relatively high torque output. In order to successfully generate power in such low speed, high torque machines, conventional power generators (designed for high speed, low torque operation) require to be connected to the turbine rotor through a gearbox. The gearbox increases the rotational speed and decreases the torque of the output from the turbine rotor which is input to the generator.
Use of gearboxes of this type is generally undesired as there are a number of significant disadvantages. In particular, the gearboxes are relatively large and heavy, greatly increasing the weight of the unit provided in the nascelle at the top of the wind turbine tower. Additionally, provision of a gearbox between the output shaft of the turbine rotor and the input shaft of the generator reduces the efficiency of the machine.
Furthermore, these gearboxes have been found to be surprisingly unreliable under typical wind turbine operating conditions. The main cause for this is the constant variation in the operating speed and torque transmitted through the gearbox, due to fluctuations in wind velocity.
Similar problems have been experienced in power generation systems using wave and tidal forces, where the prime movers of the systems operate at even lower rotational or cycling velocities, and hence at still higher torques or thrust forces.
To address these problems, different types of power generators have been developed which are designed for low speed, high torque operation, for direct connection to, for example, the rotor of a wind machine.
Examples of these types of generator include conventional permanent magnet generators, and high force density machines such as the Transverse Flux Machine (TFM) and the Vernier Hybrid Machine (VHM) of Newage AVK SEG, which have been proposed for direct drive systems. A particular application of these generators which has been identified is in wave power machines. A linear VHM machine includes opposed magnetic flux conducting cores which are generally C-shaped in cross-section, with a number of successively oppositely polarized pairs of magnets arrayed on arms of the cores, on either side of an air gap between the two opposed cores. A translator with upper and lower castellated surfaces is arranged in the air gap and is coupled to the prime mover of the power generating machine. In use, the translator reciprocates back and forth within the air gap and as the castellated portions of the translator successively align with oppositely polarized pairs of magnets, the magnetic flux flow between the two cores switches, the frequency of this switching depending upon the velocity of reciprocation of the translator. Coils are arranged on arms of the cores and thus power is generated as the magnetic field switches.
Machines of this type, those with an iron core on the stationary and moving members, suffer from significant disadvantages, particularly in that extremely large magnetic attraction forces exist between the two cores. This requires a very large and heavy support structure for the cores, in order to maintain the air gap, with a resultant effect upon the size and weight of the generator. Additionally, manufacture and assembly of the generator, due to these large magnetic attraction forces, is extremely difficult.
In an effort to address problems such as those associated with the aforementioned iron cored machines, a low force density generator, disclosed in International Patent Application Number PCT/GB02/02288, has been proposed. The generator disclosed in PCT/GB02/02288 is designed for use with a wind turbine, and is therefore a rotary generator. In the disclosed generator, the iron on the stator of the generator has been removed, and coils on the stator are supported by a non-magnetic material. In this machine, flux coming out of the moving iron surface of an iron core on a rotor of the machine has no iron surface to flow into, hence the magnetic flux effectively sees an infinitely large magnetic air gap. The flux density is therefore relatively low, and the efficiency and effectiveness of the machine is significantly reduced when compared with other generators. Accordingly, significantly more magnetic material is required on the rotor in order to achieve anything like a suitable operating efficiency. As a result, the physical diameter of the machines is required to be greatly increased. For example, for a 5MW air-cored machine, it is estimated that the machine would be 26 meters in diameter, being of the order of two to three times the diameter of an equivalent iron-cored generator.
In an alternative type of rotary generator, two iron discs are located in opposition with an air gap therebetween, and with an air cored winding sandwiched between the two moving discs. Magnets are provided on the iron discs, with successive pairs of magnets (in a circumferential direction) being of opposite polarity. When the discs rotate, the stationary windings successively experience a switching magnetic flux flow, thereby generating electricity.
However, machines of this type have extremely large magnetic attraction forces between the two discs, presenting problems of requiring a large and heavy support structure of the type described above. This presents a particularly difficult problem during manufacture of these relatively large machines, as it is extremely difficult to maintain the small air gap required (in order to maximize flux density) whilst keeping the iron discs apart.
It is therefore amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the forgoing disadvantages.